An OFDM method is a method that has been widely adopted as the standard in the fourth generation communication field, such as IEEE 802.11 and IEEE 802.16, including a broadcasting field.
Further, it is well known that synchronization of terminals in the OFDM-based multiple accesses is very important. If non-synchronized terminals simultaneously transmit data through different subcarrier waves, interferences ICI occur between neighboring subcarriers to deteriorate the communication efficiency.
Accordingly, in the MAC layer of IEEE 802.16a standard (WiMax), as illustrated in FIG. 1, ranging sub-channels for synchronization of the terminals have been prescribed.
The ranging sub-channels are provided so that (i) a terminal that firstly accesses the system obtains the synchronization, (ii) the synchronization is periodically re-adjusted even while the service is being provided, and (iii) the ranging sub-channels are used when the terminal requests resource allocation from a base station. The synchronization is performed to match the timing synchronization of received signals of all terminals by estimating timing offsets of the signals received from the respective terminals and adjusting the transmission time points of the respective terminals put in different positions based on the result of estimation.
However, there is a problem that the use of the ranging sub-channels is not always made successfully. For example, if a plurality of users intend to simultaneously perform symbol transmission through the ranging sub-channels or intend to perform symbol transmission with partial overlapping, such intentions may all fail.
As the system users are increased, such problems become severer.
To cope with this, two persons including Hisham A. Mahmoud have announced an algorithm that can improve the transmission efficiency when an initial ranging is attempted through a treatise “An efficient initial ranging for WiMAX (802.16e) OFDMA” announced in “2009 Computer Communications”. Although this algorithm presented in the treatise results in the great improvement as compared with the WiMAX protocol in the related art, it shows simulation results in that a transmission failure of about 25% occurs when 10 users randomly attempt initial ranging.
Considering that portable internet users are explosively increasing due to popularization of smart phones, overhead for synchronization of terminal is expected to be further increased.
Further, since if the initial ranging task fails, the symbol transmission for synchronization is to be continuously attempted until the time synchronization matches, the overhead for the initial ranging work to match the time synchronization is accumulated and is further increased.
This problem appears in the IEEE 802.11 WLAN that is a system using a CSMA/CA MAC mechanism as the MAC layer in addition to the WiMax using the OFDMA as the MAC layer.
That is, the CSMA/CA MAC mechanism enables only one user to use the resources during the symbol period, senses whether the transmission channel is in use, and permits competitive use of the resources.
Accordingly, the CSMA/CA MAC mechanism is also difficult to solve the users' demands explosively increasing, and overhead accompanying this becomes quite large.